The improvements in urea manufacturing plants have allowed, until now, the lowering of the steam consumption from 1500 to 1000 Kg per ton of urea or, in the best case, 550-600 Kg/t; U.S. Pat. No. 4,208,347 deals with a new, double loop, process (I.D.R process), but describes only the high pressure loop without detailing much about the final, low pressure, unit (the so called "finishing").
The Applicant [See: "New process gives urea with less energy" (Hydrocarbon Processing, Nov. 1982)] described later an I.D.R. complete manufacturing line, "finishing" included, comprising a high pressure loop (reactor, two subsequent isobaric strippers and a carbamate condenser) and a "finishing" comprising:
I-a medium pressure still (20-30 ata), the vapors of which are condensed within a vacuum concentrator, substantially at 0.35 ata (see item II), thus realizing a multiple-effect evaporation, and a low pressure still, working substantially at 5 ata (atmospheres, absolute). PA0 II-a two-step vacuum concentration section (respectively--and approximately--at 0.35 and 0.05 ata) and one unit for the recovery of NH.sub.3 from the reaction waters and from the scrubber waters coming from the washing of the synthesis vent. PA0 (a) the NH.sub.3 :CO.sub.2 ratio, in the solution fed to the CO.sub.2 stripper, is from 3 to 6 (preferably 4 to 5.5) b.w.; PA0 (b) heat is supplied only to the uppermost portion of the tubes of the CO.sub.2 stripper, wherein the ratio between the surface of the lowermost portion and the surface of the uppermost portion of the tubes is at least 2:1 (preferably from 2:1 to 4:1) and wherein the ratio between the residence time of the solution to be stripped in said lowermost portion and the residence time in said uppermost portion is at least 2:1 (preferably from 2:1 to 4:1), said residence time in the lowermost portion of the tubes being at least 3.5 (preferably from 3.5 to 5) s and said residence time in the uppermost portion being from 1.5 to 2.5 s.
The steam consumption of the complete I.D.R. line was slightly lower than 500 Kg/t and a further decrease to 500 Kg/t was forecast.
The present invention relates to an improved process for the manufacture of urea, which process permits much lower steam consumption of said I.D.R. line in a new and original way; in other words the high pressure loop and the finishing unit were first modified and then matched in an original way, so as to improve synergistically the whole I.D.R. line, while lowering the (outside) steam requirement to heretofor unimaginable levels.